Gravity drainage is a known technique for the in situ extraction of hydrocarbons. At present it is mainly performed by the injection of steam into the hydrocarbon bearing formation. However, it may also be performed by the injection of a solvent. Essentially, in a gravity drainage extraction process, the steam or solvent vapour is injected into a formation from a generally horizontal injection well and recovered from a lower parallel-running generally horizontal production well. An extraction chamber gradually develops in the formation as the oil or bitumen is removed from the reservoir above and between the wells. As the vapour flows towards the perimeter of the chamber, it encounters lower temperatures. These temperatures result in a condensation of the vapour and the transfer of heat to the sand and bitumen, causing the bitumen to warm up. In a solvent based process the warmth reduces the viscosity of the bitumen, thereby allowing the solvent to penetrate rapidly into the bitumen. The mobilized bitumen and the liquid solvent drain towards the bottom of the chamber and are then recovered from the formation through the production well located near the bottom of the chamber. As the mobilized bitumen drains downward, fresh bitumen becomes exposed at an extraction interface that is subsequently exposed to the vapour, such as the condensing solvent and becomes in turn mobilized. This bitumen depleted extraction chamber is called a gravity drainage chamber.
In such gravity drainage processes, to ensure that the vapour does not short circuit directly from the injection well to the production well, the chamber is operated with a minimum liquid head over the production well. This helps to ensure that the vapour seeks out the cold condensing surface at the extraction interface. All that is required is to restrict the liquid production from the production well to ensure that the production well is always immersed in liquid solvent (or water in the case of SAGD) and bitumen so no separate vapour path out of the chamber exists.
In a prior patent application the inventors taught that the presence of other gases in a solvent vapour based gravity drainage extraction chamber that are less condensable than the solvent being used was an impediment to solvent based gravity drainage processes. Such gases tend to be left behind in the extraction chamber as the solvent condenses and thus tend to accumulate in the chamber. If the concentration becomes large enough, they act as barrier gases. The poisoning effect of such gases on the extraction process can be mitigated through the use of high purity solvent (thus reducing the amount of such non-condensable gas that is added to the chamber during the extraction process in the first place) and controlling reservoir conditions to encourage the non-condensable components to be removed in a liquid form.
There are several key advantages for solvent assisted gravity drainage over steam assisted gravity drainage, including the in situ upgrading and the ability to achieve commercially attractive extraction rates at much lower temperatures and pressures. Thus, solvent extraction provides gains in both energy efficiency and oil quality (i.e. value) along with reduced greenhouse gas emissions and water consumption. Another advantage is that the non-condensable gases tend to have much greater solubility in the warm condensed solvent than in the hot steam condensate of a SAGD, so the ability of a solvent based process to remove non-condensable gases from the chamber is typically about 5 to 20 times better. However, solvent extraction processes have not yet been proven in the field. Further, as the solvent extraction process is applied to deeper and more gassy hydrocarbon reservoirs, it becomes increasingly difficult to avoid the accumulation of non-condensable gas in the chamber and the consequent loss in extraction rate.
What is desired is an improved method for removing the non-condensables from the extraction chamber in a solvent based gravity drainage process whether such gases arise from impurities co injected with the solvent or arising naturally from the in situ hydrocarbons as they are warmed and mobilized.